Don't throttle connection creation

[mdaigle]

Approach

We will:

• remove throttling by default
• add an app context switch to revert to the legacy hard concurrency limit of 1
• maintain blocking period choices
• optionally, configure more advanced throttling via a plugin library
  • https://gist.github.com/mdaigle/2b0388866212e79d0b1aa2bffe3ab403

Why is this the right approach?

• The upgrade story is straightforward. Users who need to maintain the hard limit just add an app context switch.
• Hard limits are easier to enforce with simple synchronization primitives and won't require us to take a dependency on polly or write complicated, custom retry logic.
• Leaving space to extend the throttling capabilities via a plugin system will allow us to add this if/when we determine it is necessary. It may be that a hard limit is sufficient for most users who require throttling (it works for them today).